Before you buy any software upgrades you will need to check that your OS version will run them. Aperture needs OSX 10.7 and Lightroom 4 needs OSX 10.6.8. And you will need to check your iMac can run this version.
If you do upgrade to 10.8 then check that any older software, that may be essential to you, will still run. Lots of older software got left behind in the latest versions of OSX.
If you have a good local reseller it may be worth talking directly to them. Make sure they really are Mac savvy & not just box shifters.
Let us know where you are and you might find some local help.
DNG converts most RAW files into a common .dng format that most (including old) software can read and still have all the RAW processing options available.
It is an extra step or 2, but it is FREE and you can still batch your files into Aperture.
I am currently using a similar workaround for D800e files through a very old Mac & using CS3 and old version of Lightroom.